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The Really Easy Guide
... to choosing and using an active subwoofer
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by: Richard Morgan
Read almost any article on home theatre that's written by a reliable writer and you'll come across the terms '5.1', '6.1' and '7.1', when referring to surround sound (either in Dolby Digital or DTS surround sound formats). It's not engineering mumbo jumbo, but in fact refers to the number of channels in a surround sound system and often the type of digital technology used to create surround sound.
 Simply put, in a 5.1 system there are five powered channels of sound (front left and right, rear effects left and right and centre channel). In a 6.1 channel system there are six (front left and right, rear effects left and right, rear centre effects and the centre channel) and seven for the 7.1 option (front left and right, side/rear effects left and right, rear centre effects left and right, plus the centre channel).
It's a simple system really, but where does the '0.1' come into it? Well that represents the separate sound channel designed specifically to cater for very low bass sounds and to which a subwoofer is connected. In the distant past (as far as HT is concerned) passive subwoofers (which drained power from the A-V amplifier or receiver to generate sound) ruled the roost.
Today, active subwoofers rule and feature inbuilt amplifiers to produce better bass and remove a lot of operating stress from amplifiers at the same time (which improves overall sound as a result).
A subwoofer is usually the last speaker element to be added to a home theatre system, but a good one can enhance the performance of a system markedly. In fact once you experience home theatre with one, you'll never go back - it's an essential element of every system and can transform a plain old music system into something very good.
Subwoofers are designed to produce low bass sounds with detail and precision. They need to move a large volume of air to do this accurately and the better they move this air the better the extension and quality of the bass they produce is.
Subwoofers operate in the low end of the sound spectrum - somewhere between 150Hz at the crossover point from the main speakers down to 20Hz in superior designs (the theoretical limit of human hearing). Hearing loses sensitivity at low bass frequencies so a subwoofer has to produce responsible sonic levels to generate convincing bass sound.
If you want Godzilla to shake the rafters like at the cinema, a subwoofer is the only way you're going get it. The good news is that it can actually sound better at home than at than the cinema.
A subwoofer is box which has large drivers installed into it, along with amplifier electronics and crossover networking. Their performance and overall potential is generally closely linked to cost so essentially you get what you pay for.
The type of bass enclosure you chose is a matter of personal choice - and in part dictated by the size of your listening room. A bigger sub generally means a bigger cabinet size and larger amplifier. It can also mean lower bass with greater impact and more of it. Trouble is, you may not want a huge box in your living room.
An amplifier's power is also tied closely to performance. Simply put, the higher the power, the louder and more accurate the subwoofer will be (assuming responsible design). A larger, more powerful; sub will be able to fill larger rooms. When assign power ratings make sure they're meaningful - that is, RMS power at low distortion levels. The level controls on a sub allow you to adjust the volume of bass to suit the movie you are watching or the characteristics of your existing speakers.
Don't get stuck on ultra low frequency response of subwoofers. Rather, it's better to have slightly less extension and more bass detail. The same goes for woofer size. While a large woofer moves more air than a smaller one (going lower and louder), many manufacturers use two smaller drivers to move the same amount of air as a single large driver. This can have benefits in speeding up bass response and improving sensitivity.
Woofers also vary in the direction they fire the sound they make - down, forward or dipole (mounted either side of the box). Down-firing subwoofers can generate punchier sound, while front and side firing designs can have a softer edge, but there's no hard rule here.
Your sub should have binding post connectors. These are sturdier and more versatile that spring-loaded connectors and help maintain performance levels from the unit. Video shielding in your subwoofer will allow flexibility in placement near a television.
Other features to look for include a phase switch and automatic on-off switch. When all the speakers in a system are moving in phase the sound they generate is cohesive. The phase switch lets you synchronise the subwoofer to the other speakers in your system. Auto power switching stops overuse and burn out when there is no sound by powering down the sub and activating again only when a signal is detected.
Subwoofers produce bass sound, which is essentially non-directional in quality and as a result the opinion is that the ears can't really tell where this sound is coming from. In theory this means we can place a subwoofer anywhere in the room and everything will be hunky dory, but a subwoofer is still a sonic point source and it's location deserves consideration.
Often this is will be a position that's in line with the front speakers and television in your system (woofer forward in front firing subs). But start with your sub in the corner, play a test track. Now move it against the side wall, then rear wall and then to the in-line position, playing the same test track in each case and noting differences between the positions.
Use the position whose acoustics you prefer (it may vary between music and movie modes). Remember also that the sub's optimum position could be different for different movie genres and even individual moves.
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